This thread looks like a good place to tell my tale of woe involving my 2005 Audi A4 1.8T
I had left the A4 unused for around a month and then went to drive it. The dash board showed an ESP fault light. This is indicating that there is a fault detected in the electronic stability program that helps to maintain control of the car when it aquaplanes or enters a spin/4 wheel skid. It is not the same as the ABS or traction control functions that my A4 is also equipped with. These systems are very clever and are important safety features so I should not complain about their presence in my car BUT ………
I plugged in my clever VAGCOM OBD II diagnostics computer and read the fault codes relating to the ESP fault. The stored code indicated that the ABS system that includes the ESP processor, was receiving improbable data from the YAW sensor. That sounded like a simple case of a failed sensor so I was relatively relaxed about the finding….. how wrong I was ! The YAW sensor and ABS computer are BOSCH parts and common across many brands of car, but they have specific programming and configuration for a particular model of car to take account of engine power, brake specifications etc. I contacted Audi for a quote on a new YAW sensor for my model of A4….. the answer was almost £1K including VAT
My 2005 A4 is not worth much more than that !
I investigated the YAW sensor and availability of used ones from car dismantlers…… there are two significant issues here….
1. The YAW sensor contains VERY fragile motion detection components and must be transported like eggs to avoid damage that could prevent correct operation. If the donor sensor came from a vehicle that had been in an accident, it is considered damaged and not fit for use
You should source a used sensor from a non accident donor, such as one that suffered a cam belt failure.
2. There are many different versions of the YAW sensor, even within the VAG group of cars. You need the EXACT YAW sensor with the same firmware as the original in order for it to work with the ABS computer. This means you have to locate a YAW sensor that has the same part number and firmware revision marked on it or else it will not work. This was not initially known to me but I found out the hard way.
My Audi A4 was from the first batch of Facelifted models released in January 2005. This should not be an issue, but it was, as we shall see. I did my due diligence and investigated which YAW sensor and firmware was fitted to a 2005 A4 and where it was located in the car. It is buried deep in the central console area on the floor pan. The centre console needs to be dismantled to even see it, let alone remove it. With this in mind, I took the guidance on which YAW sensor was fitted in my car as gospel and started sourcing multiple used sensors from non crashed A4’s in the hope of receiving one intact after being in the post. I intended to carefully test its output after fitting it to make sure it behaved correctly. I bought three of the sensors and awaited their arrival. Once they arrived I began disassembling the car to access the YAW sensor mounts. Upon extracting the original sensor I discovered that it had a different part number on it to that which was supposed to be in my A4. I thought it possible that it was just a case of Audi using the same part but with a different part number. Audi often have parts that are different part numbers yet interchangeable. I fitted one of the used replacement YAW sensors and tested the car. The fault code remained the same same…. Improbable data from YAW sensor. I tried the other two YAW sensors and got the same result. It was head scratching and Google search time. I could find no references to my car having any different YAW sensor to that which I had bought as replacements so I considered the possibility that the YAW sensor wiring had an issue with a poor connection causing CAN data corruption rather than a simple short of open type no communications fault. I tested the loom and connectors between the ABS computer and the YAW sensor and all seemed fine. I then tested the power supply rail for quality and the 0V connections to the cars shell. All checked out OK. I got a bit desperate then…. I could not locate a YAW sensor in the UK that had the same part number as the one that I found originally fitted. There was one that looked identical and had a slightly earlier part number suffix than the other used units I had bought. I bought a couple to test in the hope that I was up against a firmware revision issue and the earlier unit would work. They did not
I had exactly the same error as before and this is important to note. All my attempts to fit a replacement YAW sensor resulted in a “Improbable YAW data” fault.
A lot of head scratching and Google searches followed. There is not much information on the BOSCH ABS computer and YAW sensor available to the public but I could determine that it was very important that the ABS computer and YAW sensor had the correct, compatible, firmware loaded in them, so they are a matched pair and can communicate meaningful data. I realised that the problem I was facing was likely that I had yet to find and fit a YAW sensor with the EXACT model and firmware required by my cars ABS computer. Back to AUDI I went and they offered to sell me the YAW sensor that I had already sourced used ! This made no sense at all. I discussed the situation with the dealer and it was suggested that my ABS computer might be the problem. That would cost me £3K for a new part
I bought 3 used A4 ABS modules at £15 each to experiment with. They came from scrap Audi A4 models that were 2005 and 2006 model years. I installed and configured the ABS electronics package (computer module) from the 2005 donor as it had the exact same part number and firmware as my original. No dice, the fault remained the same. I tried both the original YAW sensor and one of the original used ones I had bought. No good. I then fitted an ABS electronics package from a 2006 A4, but a lower spec car, and reconfigured it for my cars engine power, brakes etc. Upon test with the original YAW sensor I got the same fault, but when I tried one of the used YAW sensors with the 2006 ABS computer……..all worked correctly ! Whilst this was good news, I really did not want to keep the ABS electronics from an incorrect A4 in my higher performance version of the car as who knows what differences may exist in the firmware settings.
Now that I had a working ABS and ESP system to use as a base line, I dug out my MICSIG Automotive tablet DSO and used its CAN decoder to monitor the data passing from the YAW sensor. With the working sensor I could see the valid data packets heading to the ABS computer. I then installed the original YAW sensor and immediately saw failed data packets coming out of the YAW sensors….. it was producing invalid data, just as the cars OBDII had said ! I now knew that the YAW sensor was the issue and not the original ABS computer. I had to find the exact YAW sensor for my car so an investigation was undertaken. No one could supply me with that YAW sensor for an AUDI A4 because it was apparently never fitted to an A4……..it is for an Audi A6 ! This is why I kept being told I needed a different part number for an A4
Very long story short….. my 2005 A4 was the first batch of A4 cars to use the newer BOSCH ABS computer and YAW sensor kit but it was already in use on the A6. Due to supply issues, Audi fitted my car with the ABS module and YAW sensor from their A6 production line. This does not appear to be known in the dealers or spares industry so I kept getting the wrong information when asking about a replacement YAW sensor. The later A4 production used the different part number ABS module and associated later YAW sensor. The later YAW sensor firmware is incompatible with the ABS computer that was fitted to the A6 and my particular A4.
Now the challenge was to find an A6 YAW sensor with the exact part number and firmware revision that I needed…. There were none in the UK, one on eBay in Germany and one on eBay in Lithuania. The previous used, UK sourced, YAW sensors had cost around £10 to £15 as they are common. The 2005 fitment A6 YAW sensor was going to cost around £50 due to the rarity of that particular part number (it appears that this particular YAW sensor was only used for a short period of A6 production). I bought the two available A6 YAW sensors and hoped that one would make it to me intact. They both arrived and I tested them in my A4 with the original ABS computer refitted so the ABS system was returned to my particular cars factory configuration. The sensor was fitted and began talking to the ABS computer…. No faulty packets and all valid data. The ESP fault was cleared and did not return. I tested the second used A6 YAW sensor and that also worked perfectly. I now have a good spare for future insurance. I reassembled my car and took it for various test drives and carried out the ABS-ESP test sequence. This completed without issues. I plotted the YAW sensor data output using VAGCOM both at rest and whilst test driving in circles, straight lines etc. All YAW data was valid and looked correct. The YAW sensor appears to be working perfectly.
So there we have it…. Modern cars with their modern electrical/electronic problems. We know that many modern cars need to have replacement parts “married” to the car when repairing them in order for everything to be happy, but we also have the fun of sourcing parts at reasonable cost that have compatible part numbers and firmware for your particular vehicle. Audi would have apparently fitted a new ABS module and YAW sensor to fix my A4 and that would have cost around £4K (The Official Audi A4 Service Manual says to fit a new YAW sensor when this fault code occurs, and if that does not fix it, a new ABS module should be fitted) £4K is almost twice what my car is worth. Many modern cars go to the scrap yard because of electronic or electrical faults that cannot be repaired at reasonable cost. Sad times indeed and I can see it getting worse with the advent of electric cars with all their various computers and modules that are buried deep inside assemblies that are difficult to access or disassemble. Then you have to source affordable spare parts and that is not always easy.
So my otherwise great condition, low mileage (37K miles) A4 almost ended up being scrap as it cannot obtain its yearly MOT certificate when there is an ABS or ESP fault present. I got lucky, but almost needed to be Sherlock Holmes to work out what was going on and how to fix it !
Hope you enjoyed the story
Fraser
Some additional comments……
1. I bought the A4 from new, so no one had modified its build.
2. The dealer confirmed the part that is normally fitted in the 2005 Facelift A4 and the parts catalogue listed only that part. I bought that exact part in the first batch of used YAW sensors I sourced.
3. The second batch of YAW sensors that I sourced had the same part number as the first, but an A suffix instead of a B suffix. These were fitted in the pre 2005 A4 model and I thought my facelifted 2005 A4 might have the older parts still being used in it. It turns out that the YAW sensor with the A suffix is completely different and analogue output as opposed to digital ! It is for the earlier BOSCH ABS computers that had analogue YAW inputs.
4. The YAW sensor fitted in my A4 is the modern type that contains a clever processor based input interpretation system that detects exactly what the vehicle is doing in terms of motion. It is a smart, but expensive bit of kit, and the word “Sensor” does not do it justice. The ABS system cannot be fooled into ignoring the YAW sensor as it is part of a critical safety system for the car.
5. I could not have diagnosed and configured modules in the A4 without having access to VAGCOM, but this is a common situation with modern cars needing software for diagnostics and module configuration.
6. This was the first time that I have used the MICSIG ATO1002 DSO CAN Bus decoder function and I was impressed with its ability to show me that data packets were corrupt on the CAN Bus that serves the ABS computer. It also permits checking for correct CAN Bus voltage levels.
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